1. Field of the Invention
The present invention relates to a sensorless control apparatus which controls a synchronous motor by a PWM inverter, by estimating a rotational phase angle of a rotor.
2. Description of the Related Art
In a control apparatus of a synchronous apparatus having electrical saliency at a rotor thereof, a detector is needed to detect a rotational phase angle of the rotor in order to execute driving control of the synchronous apparatus. However, in the driving apparatus using the detector, there are, for example, the following problems.
First, the presence of the detector increases the volume of the entire driving system. This hinders an increase in output of the synchronous apparatus in a limited installation space. Secondly, the detector itself requires a maintenance/inspection work. This deteriorates the maintenance/inspection efficiency. Thirdly, noise, or the like, is superimposed on a signal line from the detector, leading to disturbance in detection value and degradation in control performance. Fourthly, most of detectors require power supplies for the driving thereof, and it is necessary to install a power supply which is separate from the power supply for the synchronous apparatus driving. This becomes a factor in increase in load in terms of power supply installation space, power supply wiring and cost.
For the above reasons, there has been developed a control method which estimates a rotational phase angle without using a detector, and executes driving control on the basis of the estimated rotational phase angle. This is called “sensorless control”.
The following patent document 1, for instance, proposes a method which is particularly effective in a stop/low-speed state in a control apparatus of a synchronous apparatus including such sensorless control means. In this document, in a system which drives the synchronous apparatus by a PWM inverter, a high-frequency voltage instruction, which has a sufficiently high frequency relative to the driving frequency of the synchronous apparatus, is superimposed on a control instruction of a control apparatus which controls the inverter. From a resultant high-frequency current response, a component corresponding to the superimposed high-frequency instruction is detected and processed. Thereby, an error of the rotational phase angle is obtained. The rotational phase angle is estimated by using this error of the rotational phase angle.
The following patent document 2 proposes a method which solves a problem in patent document 1. In this method, a high-frequency component of an electric current, which occurs due to the voltage that is output from a PWM inverter, is calculated, and a rotational phase angle is estimated by using a spatial distribution of inductance.
Patent document 1: Registered Japanese Patent No. 3168967
Patent document 2: Japanese Patent Application No. 2006-185552.